One example related to the present invention includes a radio communication in an environment as shown in FIG. 1.
In a communication system 1 as shown in FIG. 1, there are a large number of terminals 100a-1 to 100a-N within radio range of an access point 201.
As a first related example, in normal wireless LAN communications such as IEEE802.11a, 802.11b, and 802.11g, the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) method is employed in a Media Access Control (MAC) layer. In the CSMA/CA method, communication is carried out in the order of 1) carrier wave sensing, 2) multiple access, and 3) collision avoidance.
In the carrier wave sensing, the terminal attempts to perform reception once before starting the communication to check whether there is another host that is currently performing communication. In a multiple access, the terminal starts the communication if there are no other terminals sharing the same channel that are currently performing communication. When there is another terminal that is currently performing communication at the stage of the carrier wave sensing, the terminal waits for a random length of time after detecting termination of communication of the terminal that has been performing the communication and then attempts to perform transmission again. This random length of waiting time gradually decreases to prevent a situation in which performance of transmission is permanently not possible.
When a plurality of terminals simultaneously perform transmission under such a circumstance, radio wave interference occurs, which results in a failure of data transmission/reception. The radio wave interference will be described with reference to FIG. 2. The radio wave interference can be mainly classified into two types: adjacent interference and hidden-terminal interference. The adjacent interference occurs when radio terminals (a terminal A and a terminal B) that are within the same communication range simultaneously start transmission. In such a case, neither of these radio terminals can receive data that has been sent. The hidden-terminal interference is generally called a hidden-terminal problem, which occurs when radio terminals (a terminal C and a terminal E) that are not within the same communication range simultaneously start to transmit data to a radio terminal D. The radio terminal D receives neither data from the terminal C nor data from the terminal E.
Since the radio wave interference described above occurs due to the presence of other radio terminals that attempt to perform transmission simultaneously, the probability that the radio wave interference occurs increases with an increase in the number of radio terminals or an increase in the traffic density of the radio communication. FIG. 3 (cited from Non-Patent Literature 1) shows a graph showing a relation between the number of radio terminals and the total throughput. The graph in FIG. 3 shows that the total throughput decreases with the increase in the number of radio terminals.